The known apparatus comprises a number of predetermined angular positions, as a rule at 15-degree intervals, and the snap engagement in the selected position is obtained by engaging a pawl into a notch of a suitable crown.
Other manufacturers obtained this indexing function by means of balls urged by springs into holes disposed at spaced intervals along the periphery of a circular plate.
The known devices have in common the same difficulties to be solved:
the manufacture of precision notched wheels; PA1 the elimination of operating play.